U.S. patent application number 14/763059 was filed with the patent office on 2015-12-10 for water intake installation for cooling a nuclear power plant, and nuclear power plant comprising such an installation.
The applicant listed for this patent is ELECTRICITE DE FRANCE. Invention is credited to Christophe Legendre.
Application Number | 20150357064 14/763059 |
Document ID | / |
Family ID | 48224979 |
Filed Date | 2015-12-10 |
United States Patent
Application |
20150357064 |
Kind Code |
A1 |
Legendre; Christophe |
December 10, 2015 |
WATER INTAKE INSTALLATION FOR COOLING A NUCLEAR POWER PLANT, AND
NUCLEAR POWER PLANT COMPRISING SUCH AN INSTALLATION
Abstract
Water intake installation comprising a suction basin from which
a pumping station supplies water to a cooling circuit, and a
suction tunnel that supplies water to the suction basin so as to
maintain a sufficient water level. The water intake installation
furthermore comprises a system for supplying additional water, able
to supply water to the suction basin from an emergency water
reserve. The system for supplying additional water comprises a
water duct connecting the suction basin to said emergency water
reserve and an obstructing device able to open the water duct if
the water level in the suction basin drops in a way defined
beforehand as being abnormal. Nuclear power plant comprising such a
water intake installation, especially suitable for establishment on
a coastline vulnerable to tsunami flooding.
Inventors: |
Legendre; Christophe;
(Virandeville, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ELECTRICITE DE FRANCE |
Paris |
|
FR |
|
|
Family ID: |
48224979 |
Appl. No.: |
14/763059 |
Filed: |
January 22, 2014 |
PCT Filed: |
January 22, 2014 |
PCT NO: |
PCT/FR2014/050123 |
371 Date: |
July 23, 2015 |
Current U.S.
Class: |
376/405 |
Current CPC
Class: |
Y02E 30/00 20130101;
G21D 3/04 20130101; E21D 9/14 20130101; E02B 5/08 20130101; E02B
5/00 20130101; G21D 1/00 20130101; Y02E 30/30 20130101 |
International
Class: |
G21D 3/04 20060101
G21D003/04; G21D 1/00 20060101 G21D001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 25, 2013 |
FR |
1350674 |
Claims
1. A water intake installation for at least one heat
exchanger-based cooling circuit of a nuclear power plant,
comprising: a suction basin from which at least one pumping station
of the plant draws water in order to circulate it within one said
cooling circuit; and at least one suction tunnel connected to at
least one main water intake submerged in a body of water, said
suction tunnel supplying the suction basin with water so as to
maintain a water level in the suction basin that is sufficient for
the operation of said at least one pumping station; wherein the
water intake installation further comprises a system for supplying
additional water distinct from said at least one suction tunnel and
capable of supplying water to the suction basin from at least one
emergency water reserve, said system for supplying additional water
comprising at least one water duct connecting the suction basin to
said emergency water reserve and an obstructing device closing off
said water duct, the obstructing device being able to open said
water duct at least partially if the water level in the suction
basin drops in a manner defined beforehand as abnormal, so that the
suction basin is supplied with water by said system for supplying
additional water if the water supplied by said at least one suction
tunnel becomes insufficient.
2. The water intake installation according to claim 1, wherein said
body of water constitutes one said emergency water reserve.
3. The water intake installation according to claim 2, wherein said
body of water is a sea, and said system for supplying additional
water is arranged between the suction basin and a portion of a
channel which communicates with the sea.
4. The water intake installation according to claim 2, wherein said
system for supplying additional water comprises a backup tunnel
connected to at least one backup water intake submerged in said
body of water, said backup water intake being placed at a height at
least ten meters above one said main water intake.
5. The water intake installation according to claim 1, wherein one
said at least one emergency water reserve comprises a reserve basin
containing a volume of water which remains substantially unchanged
when water is being supplied normally to the suction basin by said
at least one suction tunnel.
6. The water intake installation according to claim 1, wherein said
at least one main water intake is placed at a certain depth
relative to a mean reference level of said body of water, said
depth being determined such that the water flowing into the suction
basin has, during at least one period of the year, a maximum
temperature at least 4.degree. C. lower than the maximum
temperature of the water at the surface of said body of water.
7. The water intake installation according to claim 1, wherein said
obstructing device comprises an obstructing member able to pivot
about a pivot shaft in order to open said water duct.
8. Water The water intake installation according to claim 7,
wherein said obstructing device is adapted so that the pivoting of
said obstructing member occurs autonomously according to a drop in
the water level in the suction basin.
9. The water intake installation according to claim 7, wherein the
pivoting of said obstructing member is actuated by a trigger device
connected to a control system able to generate a trigger command
for the trigger device, the control system being associated with an
analysis system receiving data provided by a device for measuring
the water level in the suction basin, said analysis system being
able to determine whether the water level in the suction basin is
dropping in a manner defined beforehand as abnormal.
10. The water intake installation according to claim 9, wherein
said obstructing device is adapted so that the pivoting of said
obstructing member occurs autonomously according to a drop in the
water lever in the suction basin and wherein said trigger device is
adapted to allow the pivoting of said obstructing member to be
performed autonomously by said obstructing device if the trigger
device does not perform its function.
11. The water intake installation according to claim 8, wherein
said obstructing member pivots to open said water duct when a
height difference between the water level in the emergency water
reserve and the water level in the suction basin exceeds a
predetermined threshold.
12. The water intake installation according to claim 8, wherein
said obstructing device comprises a counterweight means arranged on
a side opposite the obstructing member relative to said pivot
shaft, said counterweight means comprising a main counterweight
member located at a fixed distanced from said pivot shaft, and said
main counterweight member weighing between 80% and 200% of the
weight of said obstructing member.
13. The water intake installation according to claim 8, wherein
said obstructing member comprises a float device arranged so that
it is fully submerged in water when water is being supplied
normally by said at least one suction tunnel and so that it is at
least partially exposed if the water level in the suction basin
falls below a predetermined level of lowest tide to reach a
predetermined trigger level, said float device being adapted to
cause said obstructing member to pivot when said trigger level is
reached.
14. A nuclear power plant comprising the water intake installation
according to claim 1, wherein the suction basin is covered by a
device forming a substantially watertight cover, and at least one
calibrated opening is made in the cover device or nearby to allow a
limited flow of water to outside the suction basin if the suction
basin overflows due to an unusual rise in said body of water, the
nuclear power plant further comprising at least one discharge shaft
feeding water to an outflow tunnel, said discharge shaft also being
provided with a cover device having at least one calibrated opening
to allow a limited flow of water to the outside in case of overflow
of the discharge shaft.
15. The nuclear power plant according to claim 14, wherein one said
emergency water reserve comprises a reserve basin having its top
open to the outside and containing a volume of water that remains
substantially unchanged when water is being supplied normally to
the suction basin by said at least one suction tunnel, and wherein
said at least one calibrated opening leads to said reserve basin to
allow collecting said limited flow of water therein.
Description
[0001] The invention relates to a water intake installation for at
least one heat exchanger-based cooling circuit, comprising a
suction basin supplied with water and from which at least one
pumping station of the plant draws water in order to circulate it
within one said cooling circuit, and further comprising at least
one suction tunnel connected to at least one main water intake
submerged in a body of water such as a sea, lake, or river, said
suction tunnel supplying the suction basin with water so as to
maintain a water level in the suction basin that is sufficient for
the operation of the pumping station.
[0002] The heat exchanger-based cooling circuit is typically
designed to cool the steam exiting a turbine-generator in a
secondary circuit of a reactor of the nuclear plant, in order to
condense this steam so that water returned to the liquid state is
fed back to the steam generators of the secondary circuit. The
steam generators draw heat from a pressurized primary circuit to
cool the reactor, by heat exchange between the primary circuit and
the secondary circuit. The primary and secondary circuits are
closed systems fluid-wise, while the heat exchanger-based cooling
circuit is open and completely isolated from the secondary circuit
which in turn is completely isolated from the primary circuit. The
water exiting a heat exchanger is therefore not radioactive, and
can be drained away for example to be returned to the body of water
supplying the circuit.
[0003] A water intake installation as defined above is known,
particularly the Seabrook nuclear power plant, constructed near the
coastline in southern New Hampshire (USA) and commissioned in 1990.
The installation comprises a single suction tunnel several
kilometers long, connected to three vertical suction shafts. Each
suction shaft opens just above the seabed about fifteen meters
below the average water level, and comprises an upper portion
forming one of said submerged water intakes.
[0004] Also known, from Japanese patent application no. JP60111089A
published on 17 Jun. 1985, is a water intake installation
comprising a suction basin supplied with water by an underground
suction tunnel, the tunnel being connected to a water intake
submerged at a relatively shallow depth in the sea. The water
intake could be left exposed before a tsunami wave.
[0005] These water intake installations are not designed to handle
the admittedly unlikely situation of a critical collapse in the
suction tunnel, which would result in almost complete obstruction
of the tunnel, the consequence being the almost complete
interruption in the supply of water to the suction basin and the
risk of insufficient water supplied to the backup pumps of the
plant's pumping station. The backup pumps are typically auxiliary
pumps to supplement the pumps of a pumping station that are used
during electricity production ("production pumps"), and are
provided to supply a reduced flow to the heat-exchanger based
cooling circuit when the production pumps are shut down. These
backup pumps are intended for cooling the nuclear reactor or
reactors when they are shut down for a long or extended period.
[0006] Even if there are two suction tunnels, one cannot ignore the
possibility of a critical collapse in both suction tunnels almost
completely cutting off the supply of water to the suction basin and
therefore to the pumping station, particularly in areas of
relatively high seismic risk. Furthermore, supplying water to the
suction basin by a tunnel connected to a water intake submerged in
the sea can have the advantage of significantly lowering the
maximum temperature of the water in the suction basin compared to
the maximum temperature of the water at the surface of the sea,
this lower temperature being related primarily to the depth at
which the water intake is placed below the mean sea level. The
addition of a second suction tunnel to supplement a first suction
tunnel, in order to limit the risk of an interruption in the supply
of water to the suction basin in case of a critical collapse in the
first tunnel, involves placing the new water intakes at least at
substantially the same depth as the first water intakes, to avoid
significantly heating the water in the suction basin.
[0007] Warming the water in the suction basin does indeed result in
a decrease in the efficiency .eta. of a secondary circuit of the
plant. The efficiency depends on the temperature Tf of the cold
source, meaning the temperature of the water at the inlet to the
heat exchangers, and is defined as follows:
.eta.=(Tc-Tf)/Tc
Tc being the temperature of the heat source, meaning the
temperature of the water exiting the heat exchangers. The
efficiency .eta. therefore increases as the temperature Tf of the
cold source decreases.
[0008] Depending on the underwater topology, the necessary length
of a suction tunnel generally increases with the depth at which the
water intakes are arranged. In addition, besides the cost of
constructing an additional tunnel, the risk of a critical collapse
in the tunnel also generally increases with the tunnel length,
especially in areas at risk for major seismic events. The solution
of an additional suction tunnel to provide a more secure supply of
water to the suction basin is therefore not entirely satisfactory,
either because of the lower efficiency of the plant's secondary
circuits when the additional water intakes are not as deep, or in
terms of cost and/or safety when the additional water intakes are
deeper.
[0009] The present invention aims to provide a water intake
installation in which, when there is a critical collapse in the
suction tunnel or tunnels supplying the suction basin, water
continues to be supplied to the suction basin for at least the
backup pumps of the plant's pumping station; this installation does
not affect the efficiency of a secondary circuit of the plant
during normal operation of the plant, meaning when water is
supplied in the normal manner to the suction basin by the suction
tunnel or tunnels.
[0010] To this end, the invention relates to a water intake
installation as defined in the preamble above, characterized in
that it further comprises a system for supplying additional water
distinct from said at least one suction tunnel and capable of
supplying water to the suction basin from at least one emergency
water reserve, said system for supplying additional water
comprising at least one water duct connecting the suction basin to
said emergency water reserve and an obstructing device closing off
said water duct, the obstructing device being able to open said
water duct at least partially if the water level in the suction
basin drops in a manner defined beforehand as abnormal, so that the
suction basin is supplied with water by said system for supplying
additional water if the water supplied by said at least one suction
tunnel becomes insufficient.
[0011] With these arrangements, the water of the suction basin
generally does not mix with the water from an emergency water
reserve during normal plant operation, and therefore the efficiency
of a secondary circuit of the plant is not impacted by the presence
of an emergency water reserve. The use of an emergency water
reserve is only triggered if the water level in the suction basin
drops in a manner defined beforehand as abnormal. A drop in water
level defined beforehand as abnormal generally corresponds to a
critical collapse in one or more suction tunnels, resulting in a
lasting interruption or at least a major decrease in the supply of
water to the suction basin. Such a drop in water level may also
correspond to an exceptional drop in the body of water for a
relatively short period, as may occur for example along the
coastline in areas prone to tsunamis. The invention therefore also
can be applied to water intake installations for nuclear power
plants on the coastline where on rare occasions the sea may drop
below the level of the lowest tide, as is sometimes the case before
the first wave of a tsunami.
[0012] According to an advantageous embodiment of a water intake
installation according to the invention, said body of water
constitutes one said emergency water reserve. In this manner, the
supplying of water to the suction basin by said system for
supplying additional water can continue for an unlimited period and
with no need for pumping means to maintain the water level in the
emergency water reserve.
[0013] In other preferred embodiments of a water intake
installation according to the invention, use is made of one or more
of the following arrangements:
[0014] said body of water is a sea, and said system for supplying
additional water is arranged between the suction basin and a
portion of a channel which communicates with the sea;
[0015] said system for supplying additional water comprises a
backup tunnel connected to at least one backup water intake
submerged in said body of water, said backup water intake being
placed at a height at least ten meters above one said main water
intake;
[0016] one said at least one emergency water reserve comprises a
reserve basin containing a volume of water which remains
substantially unchanged when water is being supplied normally to
the suction basin by said at least one suction tunnel;
[0017] said at least one main water intake is placed at a certain
depth relative to a mean reference level of said body of water,
said depth being determined such that the water flowing into the
suction basin has, during at least one period of the year, a
maximum temperature at least 4.degree. C. less than the maximum
temperature of the water at the surface of said body of water;
[0018] said obstructing device comprises an obstructing member able
to pivot about a pivot shaft in order to open said water duct;
[0019] said obstructing device is adapted so that the pivoting of
said obstructing member occurs autonomously according to a drop in
the water level in the suction basin;
[0020] the pivoting of said obstructing member is actuated by a
trigger device connected to a control system able to generate a
trigger command for the trigger device, the control system being
associated with an analysis system receiving data provided by a
device for measuring the water level in the suction basin, said
analysis system being able to determine whether the water level in
the suction basin is dropping in a manner defined beforehand as
abnormal;
[0021] said trigger device is adapted to allow the pivoting of said
obstructing member to be performed autonomously by said obstructing
device if the trigger device does not perform its function:
[0022] said obstructing member pivots to open said water duct when
a height difference between the water level in the emergency water
reserve and the water level in the suction basin exceeds a
predetermined threshold;
[0023] said obstructing device comprises a counterweight means
arranged on a side opposite the obstructing member relative to said
pivot shaft, said counterweight means comprising a main
counterweight member located at a fixed distance from said pivot
shaft, and said main counterweight member weighing between 80% and
200% of the weight of said obstructing member;
[0024] said obstructing device comprises a float device arranged so
that it is fully submerged in water when water is being supplied
normally by said at least one suction tunnel and so that it is at
least partially exposed if the water level in the suction basin
falls below a predetermined level of lowest tide to reach a
predetermined trigger level, said float device being adapted to
cause said obstructing member to pivot when said trigger level is
reached.
[0025] The invention also relates to a nuclear power plant
comprising a water intake installation according to the invention,
wherein the suction basin is covered by a device forming a
substantially watertight cover, and at least one calibrated opening
is made in the cover device or nearby to allow a limited flow of
water to outside the suction basin if the suction basin overflows
due to an unusual rise in said body of water, the nuclear power
plant further comprising at least one discharge shaft feeding water
to an outflow tunnel, said discharge shaft also being provided with
a cover device having at least one calibrated opening to allow a
limited flow of water to the outside in case of overflow of the
discharge shaft.
[0026] According to an advantageous embodiment of such a nuclear
power plant, one said emergency water reserve comprises a reserve
basin having its top open to the outside and containing a volume of
water that remains substantially unchanged when water is being
supplied normally to the suction basin by said at least one suction
tunnel, and said at least one calibrated opening leads to said
reserve basin to allow collecting said limited flow of water
therein.
[0027] Other features and advantages of the invention will be
apparent from the following description of some non-limiting
exemplary embodiments, with reference to the figures in which:
[0028] FIG. 1 schematically represents a top view of a nuclear
power plant near the coastline, comprising a water intake
installation able to be modified to equip it with a system for
supplying additional water.
[0029] FIG. 2 schematically represents a partial side view of the
water intake installation represented in FIG. 1, as well as the
different tide levels to be taken into account in the design.
[0030] FIG. 3 schematically represents a top view of the nuclear
power plant of FIG. 1, in a situation with highly degraded
operation of the suction tunnel after a collapse; this situation
does not allow the plant to continue operating normally.
[0031] FIG. 4 schematically represents a partial side view of
modifications made to the water intake installation of FIG. 1 in
order to implement a system for supplying additional water
according to the invention, with the obstructing device of the
system represented in a position where it closes off the water
duct.
[0032] FIG. 5 represents the system for supplying additional water
of FIG. 4, with the obstructing device in a position that opens the
water duct, placing the suction basin in communication with a
channel.
[0033] FIG. 6 schematically represents a partial top view of the
system for supplying additional water of FIG. 4.
[0034] FIG. 7 schematically represents a partial top view of the
system for supplying additional water of FIG. 4, with the
obstructing device in the open position of FIG. 5.
[0035] FIG. 8 schematically represents a partial side view of a
portion of the obstructing device of FIG. 4.
[0036] FIG. 9 schematically represents a partial side view of the
obstructing device of FIG. 8 plus a counterweight adjustment
means.
[0037] FIG. 10 schematically represents a partial side view of an
obstructing device similar to the one of FIG. 9.
[0038] FIG. 11 schematically represents a partial side view of
another embodiment of a system for supplying additional water of
the invention, which can be used as an alternative to the system
for supplying additional water of FIG. 4.
[0039] FIG. 12 represents the system for supplying additional water
of FIG. 11 with the obstructing device in a position that fully
opens the water duct.
[0040] FIG. 13 schematically represents a partial side view of a
variant of the system for supplying additional water of FIG. 11,
with the obstructing device in a position that closes off the water
duct.
[0041] FIG. 14 schematically represents the system for supplying
additional water of FIG. 13, with the obstructing device in a
position that fully opens the water duct.
[0042] FIG. 15 schematically represents a partial side view of
another variant of a system for supplying additional water similar
to that of FIG. 11, with an obstructing device according to another
embodiment.
[0043] FIG. 16 represents the system for supplying additional water
of FIG. 15, with the obstructing device in a position that fully
opens the water duct.
[0044] FIG. 17 schematically represents a partial side view of
another embodiment of a water intake installation of the invention
for a nuclear power plant that could experience a tidal wave, the
obstructing device of the water supply system being represented in
a position that closes off the water duct.
[0045] FIG. 18 represents the system for supplying additional water
of FIG. 17, the obstructing device being in a position that opens
the water duct so that the suction basin communicates with the sea
via a backup tunnel.
[0046] FIG. 19 schematically represents a partial side view of the
system for supplying additional water of FIG. 17, equipped with an
obstructing device according to another embodiment.
[0047] FIG. 20 schematically represents a partial side view of
another embodiment of a water intake installation of the invention,
for a nuclear power plant by the coastline that could experience a
tidal wave, with a first emergency water reserve comprising a
reserve basin particularly intended for handling a tsunami
situation.
[0048] FIG. 21 represents the water intake installation of FIG. 20
in a situation where the sea bordering the plant drops below the
level of the lowest tide prior to the first wave of a tsunami, the
reserve basin allowing the supply of water to the production pumps
to continue.
[0049] FIG. 22 represents the water intake installation of FIG. 20
in a situation where the level of the sea bordering the plant
reaches its peak during a tsunami.
[0050] FIG. 23 represents the water intake installation of FIG. 20
in a situation where the supply of water through the suction tunnel
to the suction basin is interrupted due to a collapse, the suction
basin being supplied with water indirectly by a backup tunnel in
order to maintain operation of the backup pumps.
[0051] FIG. 24 schematically represents a portion of the water
intake installation of FIG. 20, in which trigger devices are
installed to control the opening of the obstructing devices sealing
off the system for supplying additional water, one of the trigger
devices being represented as actuated to allow the reserve basin to
be filled.
[0052] FIG. 25 schematically represents another embodiment of the
water intake installation of FIG. 23, in the same situation where
the supply of water through the suction tunnel to the suction basin
has been interrupted, the suction basin being supplied with water
directly by a backup tunnel.
[0053] FIG. 26 schematically represents a front view of one
embodiment of an obstructing device with exclusively controlled
opening, usable in a water supply system of the water intake
installation of FIG. 25, the obstructing device being shown in a
position where it closes off the water duct.
[0054] FIG. 27 represents the obstructing device of FIG. 26 in an
intermediate position of unobstructing the water duct immediately
after it is triggered to open.
[0055] FIG. 28 schematically represents a partial side view of the
obstructing device of FIG. 26.
[0056] FIG. 29 represents the obstructing device of FIG. 28 in an
intermediate position of unobstructing the water duct.
[0057] FIG. 30 schematically represents a partial side view of a
modified portion of the obstructing device of FIG. 26, in a
position of closing off the water duct as well as in an
intermediate position of unobstructing the water duct.
[0058] FIG. 31 schematically and partially represents another
embodiment of a water intake installation similar to that of FIG.
20, where both the reserve basin and the suction basin are covered
by a cover device.
[0059] FIG. 32 schematically represents a partial side view of
another embodiment of a water intake installation of the invention
for a nuclear power plant separated from the water's edge by a
strip of land not suitable for construction, an emergency water
reserve comprising a reserve basin which can be supplied water from
an auxiliary water source such as a river.
[0060] FIG. 1, FIG. 2, and FIG. 3 represent the same water intake
installation and are discussed together in the following. The water
intake installation is installed at the site of a nuclear power
plant 1 on the coastline, and comprises a suction basin 2 located
in a bottom portion 63 of a channel 6, as well as an underground
suction tunnel 3 which supplies the suction basin with water. A
plant pumping station 10 pumps water into the suction basin 2 for
use in at least one heat exchanger-based cooling circuit. The
underground tunnel 3 is in communication with the suction basin 2
by means of two shafts each formed by a generally vertical passage
7 which leads to the bottom 2B of the basin, as represented in FIG.
2.
[0061] The underground suction tunnel 3 is visible in FIGS. 1 and 3
for explanatory purposes, but it is understood that this tunnel is
buried below the seabed and is therefore not visible from the sea.
The tunnel 3 extends to a certain distance from the shoreline,
passing below the bed to reach a depth below sea level (MSL in
France) that is defined beforehand based on a maximum temperature
that the water in the suction basin is not to exceed. In the
embodiment represented in FIG. 1, the suction tunnel 3 lies under
the seabed at depths of about 40 meters below mean sea level, and
is connected to two water intakes 51 and 52 spaced apart from each
other.
[0062] Each water intake 51 and 52 sits several meters above the
seabed at a depth H below mean sea level L.sub.0, and is located at
an upper end of a substantially vertical suction shaft 8 connected
to the suction tunnel as represented in FIG. 2. The water gains
very little heat in an underground suction tunnel, and therefore
the water arriving at the suction basin is substantially the same
temperature as the water collected at a water intake 51 or 52.
Preferably, the depth H is determined so that the water reaching
the suction basin 2 has a maximum temperature during at least a
period of the year that is at least 4.degree. C. lower than the
maximum surface temperature of the water constituting the body of
water 5.
[0063] In the example represented in FIG. 1, the suction tunnel 3
forms a loop having a curved section 3C forming at least a
half-circle, and has two ends which each communicate with the
suction basin 2 by means of a generally vertical passage 7. The
water intakes 51 and 52 allow the tunnel to pull water in
respective streams flowing at rates I.sub.1 and I.sub.2 that are a
function of the pumping rate of the pumping station 10. If a
reactor unit 1A at full power requires about 70 m3 per second of
water during normal operation for example, the flow rate of each
stream I.sub.1 or I.sub.2 is about 35 m3 per second of water. The
inside diameter of the tunnel 3, as well as the inside diameter of
a passage 7 and of a suction shaft 8, is chosen to be about 5
meters for example, which ensures a flow rate of 70 m3 per second
of water in one arm 3B or 3D of the tunnel without substantial head
loss in an unaffected arm if the other arm is blocked by a
collapse.
[0064] In a water intake installation according to the invention,
it is not necessary for the suction tunnel 3 to form a loop or for
only one suction tunnel 3 to supply a suction basin 2 of the plant.
Any other form of suction tunnel is possible, and a suction basin 2
can be supplied with water by two or even three separate suction
tunnels. In particular, if one suction basin is allocated to
pumping stations for multiple reactors of a plant, for safety
reasons or in order to maintain the necessary flow rate it may be
decided to have the suction basin supplied by two looped suction
tunnels 3 arranged side by side. Furthermore, in a known manner, a
pumping station comprises pumps R (see FIG. 4) for sending the
water exiting the heat exchanger 13-based cooling circuit 11 to a
discharge shaft 14 leading to an outflow tunnel 4 which ends in
underwater mouths 41 located at a distance from the water intakes
51 and 52. The flow rate I.sub.R of water discharged by the outflow
tunnel 4 is normally equal to the sum of flow rates I.sub.1 and
I.sub.2.
[0065] The channel 6 comprises an intake portion 60 which
communicates with the sea 5, and is protected from the sea by a
dike 61 between the channel and the shoreline 5B. A wall 62, for
example in the form of a dam wall, creates a separation between the
bottom portion 63 and the intake portion 60 of the channel, so that
water from the suction basin 2 does not mix with the water of the
intake portion of the channel. In this manner, water from the
suction basin 2 is not heated by the generally warmer water of the
channel 6. The wall 62 and the tunnel and suction shafts may be
constructed as part of modifications to a nuclear power plant
already in operation where the suction basin was originally formed
by the channel 6, in order to lower the maximum temperature of the
water supplied to the plant pumping station.
[0066] In the unlikely event of damage to both arms of the suction
tunnel 3, for example in areas 55 of the tunnel suffering a
critical collapse as schematically represented in FIG. 3, there
could be significant localized narrowing of the inside
cross-sectional area of the tunnel. Studies conducted by the
applicant allow one to assume that with a tunnel containing
reinforcing wall segments that can move in a direction transverse
to the tunnel, and in the most serious collapses considered, the
inside cross-sectional area of the tunnel in the damaged areas
would remain sufficient to allow a flow rate for example of at
least 5 m3 per second of water and greater than the emergency flow
rate required by the backup pumps in the pumping station 10. An
emergency flow rate of about 4 m3 per second of water is usually
enough to cover the water supply requirements of a pumping station
of a reactor unit where the generation of electricity has been
stopped.
[0067] Nevertheless, the current state of research does not allow
predicting with certainty that the inside cross-sectional area of
the tunnel would systematically remain sufficient in all possible
cases of collapse. One cannot completely rule out the possibility
of severe narrowing of the inside cross-sectional area of the
tunnel, more or less cutting off the water supply to the suction
basin 2 which means sufficient water is prevented from reaching the
backup pumps from the suction tunnel. The case of a critical
collapse as represented in FIG. 3 could therefore lead to cooling
failure of the nuclear reactor, even during reactor shutdown. For
these reasons, the applicant has sought to design a system for
supplying additional water that is capable of placing the suction
basin in communication with an emergency water reserve, said system
being intended to ensure that the supply of water to the suction
basin from the emergency water reserve is infallibly triggered
whenever the flow of water from the suction tunnel becomes
insufficient to supply the backup pumps.
[0068] In the following description, it is assumed that the body of
water 5 is a sea subjected to tides. It is understood that the
embodiment described is also suitable for a body of water having no
substantial variation in level. Each wall of the passage 7 ends at
the suction basin 2 at a level which is substantially below the
level L.sub.L of the lowest tide during the largest tidal
coefficients (see FIG. 2). Indeed, the supply of water through the
suction tunnel 3 to the suction basin is effected by the
equilibrium established between levels due to atmospheric pressure.
Taking into account the pumping rate of the pumping station 10, the
head losses in the suction shafts 8 and tunnel 3 may result in the
water level L.sub.2 in the suction basin being several centimeters
or tens of centimeters below the level L.sub.1 of the sea measured
above the water intakes 51 and 52, the level L.sub.1 in question
being averaged between the peaks and troughs of the swell waves.
This averaged level L.sub.1 is substantially the same above the
water intakes and in the channel 6, which smoothes out the rapid
variations in water level due to swells. When the level L.sub.1 of
the sea reaches the level L.sub.L of the lowest tide, the water
level L.sub.2 in the suction basin reaches a level L.sub.2L which
must be at a certain height above the mouth 7E of the passage 7, to
prevent the suction basin from being progressively emptied by the
production pumps of the pumping station 10. The height of the
suction basin is such that when the level L.sub.1 of the sea
reaches the level L.sub.H of the highest tide during the largest
tidal coefficients, water does not overflow from the suction
basin.
[0069] In the embodiment represented in FIG. 1, where the suction
basin 2 is implemented within a channel 6, the emergency water
reserve is preferably formed by the intake portion 60 of the
channel which is mostly protected from the waves and ground swells
that can be encountered outside the channel in a coastline setting.
A filtration system may be provided at the entrance to the channel,
not shown in the figure, for example comprising grills that can be
cleaned from time to time, to keep the water in the intake portion
60 of the channel free of contaminants such as floating objects or
algae. In fact, due to the fact that the water coming through the
suction tunnel 3 does not contain such contaminants, a filtration
system 12 for the pumping station 10 (see FIG. 2) can
advantageously omit the filtration and cleaning means specifically
handling these types of contaminants. In an emergency situation
where the suction basin 2 must quickly be supplied with water by
the intake portion 60 of the channel, we do not want to risk
fouling the filtration system 12.
[0070] As represented in FIG. 4 as well as in FIGS. 5 to 7, in
order to implement the system for supplying additional water, the
closed wall 62 is replaced by a partition wall 620 having an
opening 65 blocked by an obstructing device in the form of a
pivoting valve 9. The valve 9 comprises an obstructing member 90 in
the form of a sealing panel that is generally planar, for example
substantially rectangular, and pivotable about a pivot shaft 91.
The valve 9 further comprises a counterweight means arranged on a
side opposite the sealing panel 90 relative to the pivot shaft 91.
The counterweight means comprises a main counterweight member 92
located at a fixed distance from the pivot shaft 91. The
counterweight means further comprises an adjustable auxiliary
counterweight means, which comprises for example an auxiliary
counterweight 94 movably mounted on two arms 93 fixed to the valve
9. In this manner, the position of the center of gravity G of the
obstructing device 9 can be adjusted to some extent, as detailed
below with reference to FIG. 9. The valve 9 is designed such that
the center of gravity G is located at a certain distance from the
plane of the sealing panel 90, so that the torque exerted by the
weight of the valve with respect to the pivot shaft 91 provides a
force that keeps the valve closed despite the level L.sub.1 of the
sea being higher than the water level L.sub.2 in the suction
basin.
[0071] In order to have a constant pumping rate of the pumping
station 10 supplying water to a heat exchanger 13-based cooling
circuit 11, the difference in height .DELTA.h between the level
L.sub.1 of the sea and the water level L.sub.2 in the suction basin
virtually does not vary with the level of the sea. The valve 9
closing force provided by the weight of the valve as explained
above is intended to be greater than the valve opening force
required by the water pressure differential between the two faces
of the sealing panel 90 due to the difference in height .DELTA.h,
this difference .DELTA.h being considered for a pumping rate of the
pumping station during normal operation with the corresponding
reactor unit at full power. In this manner, as long as the suction
basin 2 is supplied with water by a suction tunnel 3 as normal, the
valve 9 remains closed as represented in FIG. 4 and FIG. 6, so that
there is almost no mixing of the water in the suction basin with
the water of the emergency water reserve formed by the intake
portion 60 of the channel. It is not necessary for the valve 9 to
provide a perfect seal, as it is acceptable for water to leak from
the intake portion 60 to the suction basin 2 as long as this does
not significantly increase the water temperature in the suction
basin.
[0072] The valve 9 closing force provided by the weight of the
valve is intended to correspond to a predetermined critical
difference .DELTA.hV in the water levels that unerringly indicates
an insufficient supply of water to the basin 2 via the suction
tunnel or tunnels 3. In other words, it is arranged that the valve
opening force resulting from this critical difference .DELTA.hV is
stronger than the valve closing force once the height difference
.DELTA.h exceeds the critical difference .DELTA.hV, causing the
valve to open once the critical difference .DELTA.hV is reached. In
practice, the static friction of the valve's pivoting elements must
also be considered, for example the bearings associated with the
pivot shaft 91 if the latter pivots on bearings 95 (see FIG. 5 and
FIG. 7).
[0073] A collapse in a suction tunnel 3 is unlikely to occur
precisely during a period when the level L.sub.1 of the sea is as
low as the level L.sub.L of the lowest tide during the strongest
tidal coefficients. As a result, if the critical height difference
.DELTA.hV is reached after a collapse in the tunnel, the valve 9
will generally open while the water level L.sub.2 in the suction
basin 2 is still above a critical level L.sub.2V corresponding to
the case of the lowest tide indicated in FIG. 5.
[0074] Furthermore, the sizing of the valve 9 may vary depending on
the desired function of the system for supplying additional water.
It may be desired to allow the water to travel through the valve 9,
once it is open, at a rate sufficient to allow normal operation of
the pumping station 10 for a reactor unit generating electricity at
full capacity during periods where the water temperature at the
surface of the sea does not exceed a certain value, for example
between 10.degree. C. and 20.degree. C. The repair of a suction
tunnel having experienced a collapse may take months or even more
than a year for a critical collapse in several arms of the tunnel.
Electricity generation by the nuclear power plant could then be
continued during some or all of the work period, particularly in
the winter, by using the channel 6 to supply water to the suction
basin 2. As an alternative to a valve 9 of large dimensions to
accommodate the maximum flow rate required for electricity
generation, a valve 9 of smaller size can be provided, arranged in
parallel with a main gate valve such as a raising gate installed
beside valve 9 within the partition wall 620. The main gate valve,
not shown in the figures, would be controlled to open after valve 9
is triggered, the opening of the gate valve being required in order
to restart the production pumps.
[0075] In other configurations of nuclear power plants, for example
in the case of a nuclear power plant installed near a sea that
remains relatively warm year round, normal operation of the pumping
station 10 to generate electricity at full capacity may be
impossible if the water must be supplied to the suction basin
through the channel 6. In this case, valve 9 can have relatively
small dimensions that allow sufficient water through to achieve a
minimum flow rate, for example about 5 m3 per second, for reliably
supplying the backup pumps of the pumping station 10 with the water
required. It is also conceivable for valve 9 to have sufficient
dimensions for supplying the production pumps with a reduced flow,
in a context of reduced electricity production by the plant.
[0076] The dimensions of the suction basin 2 should take into
account the extreme case where a critical collapse in the suction
tunnel 3 occurs during a period when the level L.sub.1 of the sea
has reached the level L.sub.L of lowest tide during the strongest
tidal coefficients. Just before the supply of water from the
passages 7 connected to the suction tunnel is cut off, the water
level L.sub.2L in the suction basin is at a height below level
L.sub.L. Once the water supply is cut off or is at least
insufficient for the water consumed by the pumping station 10, a
more or less rapid drop in the water level in the suction basin
occurs, to reach the critical level L.sub.2V as shown in FIG. 5. As
explained above, the valve 9 is then forced to pivot open. In
addition, a system for detecting the water level and/or the
pivoting of the valve 9 may advantageously be provided, for forcing
shutdown of electricity production and a switch from the production
pumps of the pumping station 10 to the backup pumps.
[0077] The filtration system 12 is arranged below the critical
level L.sub.2V, and the water intakes of the pumping station 10 are
arranged sufficiently below this level to avoid their exposure as
the water level in the suction basin continues to drop during the
shutdown phase of the production pumps. Depending on the flow rate
of the water through the open valve 9, the water level in the
suction basin will climb back more or less quickly, and at the
latest once the production pumps have completely stopped. Thanks to
the counterweight means of the valve 9, the positioning of the
center of gravity G of the obstructing device above the level of
the pivot shaft 91 allows the torque exerted by the weight of the
valve about the pivot shaft 91 to decrease as the valve opens. As a
result, the valve remains open in a position of dynamic equilibrium
which is maintained when the height difference .DELTA.h of the
water is once again less than the critical difference
.DELTA.hV.
[0078] The valve 9 described above is an obstructing device in
which the pivoting occurs autonomously, meaning in a passive manner
without requiring an external device to trigger it. Optionally, the
pivoting of the valve 9 can be actuated by a trigger device
connected for example to a control system associated with a water
level detection system. The trigger device may, for example, act on
a cable connected to a crank attached to the valve at the pivot
shaft 91, and may advantageously be adapted to allow the valve to
pivot autonomously in the event that the trigger device does not
function. The trigger device may also be arranged to maintain the
valve 9, after it is triggered, in a position where it is more
widely open than in the dynamic equilibrium position mentioned
above with reference to FIG. 5.
[0079] As represented in FIG. 6 and FIG. 7, the auxiliary
counterweight 94 may be formed by a beam structure mounted to be
slidable perpendicularly to two arms 93 parallel to each other, in
a manner that adjusts the distance between the beam 94 and the
pivot shaft 91 parallel to it. In addition, the opening 65 forming
the water duct in the wall 620 separating the suction basin 2 from
the intake portion 60 of the channel may be provided with a
filtration and/or safety grid on the intake portion 60 side.
[0080] Advantageously, the main counterweight member 92 weighs
between 80% and 200% of the weight of the obstructing member 90. In
this manner, as represented in FIG. 8, the center of gravity G1 of
the assembly of the two members is relatively close to the pivot
shaft 91 within a height range DG1. To raise the position of the
center of gravity G1, the weight of the main counterweight 92 can
be increased and/or the position of its center of gravity raised.
The auxiliary counterweight means attached to this assembly is
arranged such that the center of gravity G of the entire assembly
is located above the level X of the pivot shaft 91, as represented
in FIG. 9. Adjusting the position of the auxiliary counterweight 94
in a direction A1 within a certain margin DG2 moves the center of
gravity G2 of the auxiliary counterweight means, and therefore
moves the center of gravity G more or less further away from the
pivot shaft 91. Thus, if during testing or normal operation the
valve 9 is opened unexpectedly while the suction tunnel is
functioning, for example during a storm hitting the coastline 5B,
the position of the auxiliary counterweight 94 can be readjusted to
correspond to a critical height difference .DELTA.hV that has been
reevaluated upward.
[0081] The main counterweight member 92 and the device comprising
the auxiliary counterweight 94 may form an assembly that is a
single piece for all intents and purposes, which is secured to the
obstructing member 90 by fitting it thereon, as represented in FIG.
10.
[0082] Another embodiment of a system for supplying additional
water is represented in FIGS. 11 to 14, for a water intake
installation according to the invention. In comparison to the
previous embodiment, this embodiment allows reducing the dimensions
of the obstructing device 9, and in particular the dimensions of
the obstructing member 90. As represented in FIG. 11 and FIG. 12,
the opening 65 forming the water duct in the wall 621 separating
the suction basin 2 from the intake portion 60 of the channel is
arranged in a lower portion of the wall 621. A sealing panel that
is generally planar, for example substantially rectangular, forms
the obstructing member 90 of the valve 9. The dimensions of the
sealing panel 90 are somewhat larger than the cross-sectional area
of the passage of the opening 65, said cross-sectional area
possibly being relatively small, for example about 2 to 3 m2, to
permit only the passage of enough water to supply reliably the
backup pumps of the pump station 10. As explained for the previous
embodiment, it is also possible to arrange in parallel a main gate
valve such as a sliding gate valve actuated by a control, also
known as a slice valve, installed beside valve 9 in the partition
wall 621.
[0083] The pivot shaft 91 of the valve 9 is attached at a lower
edge of the sealing panel 90. The pivot elements of the valve
comprise, for example, the bearings associated with the pivot shaft
91 and arranged to rotate on bearing mounts on the bottom of the
suction basin. Pneumatic caissons or hollow watertight columns may
be provided, each having a wall traversed by the pivot shaft 91, in
order to contain the bearings and mounts and surround them with
air. As an alternative to the bearings, it may be arranged that the
pivot shaft 91 is formed by a bar having a ridge for example of
stainless steel along its length, which presses against the inner
surface of a half-tube or similar bearing element having a concave
face parallel to the bar and attached to the ground at the bottom
of the basin. The concave face of the bearing element will
generally be oriented towards the intake portion 60 of the channel,
to prevent movement of the pivot shaft 91 in the direction of the
suction basin including after the valve 9 has pivoted as
represented in FIG. 12. The static friction of such a device with
its ridged pivot shaft can be fairly low, and in particular can be
relatively stable over time without requiring special maintenance
of the device.
[0084] The sealing panel 90 is installed within the opening 65 in
the wall 621 so as to seal the opening in a more or less fluidtight
manner, and is mounted with a certain inclination relative to the
vertical direction. An abutment maintaining the inclined position
of the panel 90 is formed for example by a shoulder 622 of the wall
621. The inclination and weight of the panel 90 are defined
beforehand so that the panel remains in position during situations
of normal operation of the suction tunnel, as shown in FIG. 11. In
other words, the panel 90 must not pivot under normal conditions,
despite the differential water pressure on the face of the panel on
the channel side due to the height difference .DELTA.h between the
level L.sub.1 of the sea and the water level L.sub.2 in the suction
basin, but must pivot to open the valve 9 if the critical height
difference .DELTA.hV is reached as represented in FIG. 12.
[0085] The valve 9 does not require a massive counterweight member
such as the main counterweight member 92 described above. In fact,
once the panel 90 begins to pivot, the inclination of the panel
relative to the vertical direction decreases, which reduces the
torque exerted by the weight of the panel relative to the pivot
shaft 91 and therefore decreases the resistance of the valve to the
opening force caused by the critical height difference .DELTA.hV.
The valve 9 is therefore certain to open fully when the panel 90
starts to rotate.
[0086] In FIG. 13, a variant of the system for supplying additional
water of FIG. 11 consists of providing the valve with an adjustable
counterweight means comprising, for example, a counterweight 94
movably mounted on two parallel arms 93 fixed to the valve 9, in a
manner analogous to the auxiliary counterweight means 94 described
above in reference to FIG. 4 and FIG. 6. Furthermore, in order to
optimize the cross-sectional area of the opening 65 in the
partition wall 621, the floor is sunken under the counterweight 94,
and the abutment maintaining the inclined position of the panel 90
is formed near the pivot shaft 91. A relatively light counterweight
94, for example weighing less than 10% of the weight of the panel
90, can be sufficient for tests adjusting the center of gravity G
of the valve.
[0087] As represented in FIG. 13, the valve 13 is subjected to two
opposing torques, meaning torques in opposite directions relative
to the pivot shaft 91. The torque exerted by the weight of the
valve is equal to the value F1 of the weight multiplied by the
distance D1 between the weight vector applied at the center of
gravity G of the valve and the center axis C of the pivot shaft 91.
The algebraic torque exerted by the force of the differential water
pressure that is applied to the panel 90 is equal to the algebraic
value F2 of this force multiplied by the distance D2 between the
force vector F2 and the central axis C. The angle of the panel 90,
as well as the center of gravity and the weight of the valve, are
defined beforehand so that the two opposing torques have the same
absolute value if the critical height difference .DELTA.hV in the
water levels is reached. As represented in FIG. 14, when the
critical height difference .DELTA.hV is slightly exceeded this
overcomes the static friction of the device with its pivot shaft
91, causing the panel 90 to pivot which opens the valve 9. The
water level L.sub.2 in the suction basin may continue to descend as
long as the production pumps are not completely shut down, and
climbs back up when only the backup pumps are active.
[0088] Another embodiment of a system for supplying additional
water similar to the one of FIG. 11 for a water intake installation
according to the invention is represented in FIG. 15. The
implementation of the obstructing device 9 in particular is
different from the previous embodiment, especially in that the
sealing panel 90 is not the only sealing element of the valve 9
between the suction basin 2 and the intake portion 60 of the
channel. Indeed, here a main counterweight member 92 as previously
described forms a sealing surface S3 on the side of the panel 90
away from the pivot shaft 91. In this manner, torque exerted due to
the force F3 of the differential water pressure which is applied to
the sealing surface S3 is added to the torque exerted by the weight
F1 of the valve, in a direction of rotation opposing the torque
exerted by the force F2 of the differential water pressure which is
applied to the panel 90.
[0089] This implementation of the valve 9 keeps the valve closed
until there is a relatively large critical height difference
.DELTA.hV, without requiring a particularly massive counterweight
system. Indeed, the design may provide for increased dimensions of
the sealing surface S3 in order to adapt the valve for a greater
critical height difference .DELTA.hV. In addition, as represented
in FIG. 16, once the valve 9 is open it exposes a water duct having
a cross-sectional area virtually equal to the cross-sectional area
of the opening 65. In addition, depending on the intended position
of its center of gravity G, the valve may be arranged to close
autonomously if the operation of the suction tunnel is restored.
Optionally, a filtration and/or safety grid 12' may be provided on
the opening 65 on the suction basin 2 side.
[0090] A system for supplying additional water for a water intake
installation according to the invention may comprise a backup
tunnel, in particular if the suction basin is at a distance from
the emergency water reserve. This may be the case, for example, if
the nuclear power plant is separated from the sea by a section of
land where construction is not possible, thus preventing the
construction of a channel to the suction basin but allowing the
passage of a backup tunnel beneath said section of land. This may
also be the case, for example, if the power plant is located next
to a body of water likely to experience an unusual rise in water
level.
[0091] In FIG. 17, a water intake installation according to the
invention can be adapted for such a nuclear power plant located
next to such a body of water. An unusual rise in water level is
understood to mean a tidal wave such as those caused for example by
a tsunami, or floodwaters swelling a river. A water intake
installation such as the one represented in FIG. 1 requires
relatively few arrangements to withstand an unusual rise in water
level. The dike 61 must be of sufficient height to prevent flooding
if the body of water 5 reaches the height L.sub.1P of the highest
estimated level. In addition, the dike 61 must protect the plant
completely, and therefore there is no longer any question of an
opening to the sea such as a channel. To simplify the description,
it is considered in the following that the body of water 5 is a
sea, but it is understood that the installation described also
relates to any body of water suitable for cooling a plant, such as
a river for example.
[0092] Advantageously, the mouth 7E of a passage 7 connecting the
suction basin 2 to the suction tunnel 3 is located at a
predetermined height above the bottom 2B of the suction basin, so
that in the event of an exceptional drop of the sea to below the
level L.sub.L of the lowest tide, as can occur for example along
the coastline in areas prone to tsunamis, a certain volume of water
remains as a reserve in the suction basin. In the most critical
estimate of the drop in the sea level, the level L.sub.1 of the sea
will remain below the level of the mouth 7E of the passage 7 for a
certain period of time, which means that during this time, which
may last several minutes, the water to the pumping station 10 will
only be supplied from the reserve volume of water. This volume of
water must therefore be arranged so that there is time to shut down
the production of electricity by the nuclear reactor and to switch
from the production pumps of the pumping station 10 to the backup
pumps, and to do so with no risk of interruption of the water
supply to the backup pumps. It must be possible to supply the
backup pumps from the reserve volume of water until the sea rises
sufficiently for the water in the passage 7 to return to above the
level of the mouth 7E of the passage, meaning until the tunnel 3 is
again supplying the suction basin. As a first approximation, it is
estimated for example that a reserve volume of water of about
10,000 m3 for a pumping station for one nuclear unit is sufficient
to offset the most critical drop possible in the level of the sea
prior to a first wave of a tsunami, lasting at least fifteen
minutes or so.
[0093] To avoid an uncontrolled overflow of the suction basin 2
during an unusual rise in the sea, for example during or after a
first wave of a tsunami, the basin is covered by a device forming
an essentially watertight cover 25. Calibrated openings 26 can be
made in or near the cover 25, for example in a side wall of the
basin between the basin and its outside environment. In this
manner, if the basin 2 is completely filled, the calibrated
openings 26 allow a limited flow of water I.sub.p from the basin to
the outside environment. The flow I.sub.p may be channeled to a
small basin 22 formed on a cover of a compartment 21 of the suction
basin 2, before being discharged for example into the sea at low
tide.
[0094] In addition, as explained above with reference to FIG. 1 and
FIG. 4, in a nuclear power plant 1A the water leaving the heat
exchanger 13-based cooling circuit 11 is drained into a discharge
shaft 14 for discharge into the sea via an outflow tunnel 4. In the
event of an unusual rise of the sea, uncontrolled overflow of the
discharge shaft must be avoided. Advantageously, the discharge
shaft 14 is also provided with a cover device with at least one
calibrated opening to allow a limited flow of water to outside the
discharge shaft in the event of overflow. This arrangement applies
to any nuclear power plant comprising a water intake installation
of the invention and likely to experience an unusual rise in the
level of the body of water 5. Furthermore, in order to counter the
possibility of a relative blockage of the outflow tunnel 4, the
discharge shaft 14 may advantageously be provided with a closed
valve which opens to the outside only beyond a certain water
pressure in the shaft, or an obstructing device which is controlled
to open so that it is in communication with an auxiliary outflow
passage leading to the sea. In the event of blockage of the outflow
tunnel 4, the water level in the discharge shaft 14 will rise due
to the water contributed by the pumps R (FIG. 4), and the valve or
the obstructing device is triggered to open shortly before the
level reaches the top of the shaft in order to drain the water away
by the auxiliary outflow passage.
[0095] The maximum water pressure in the suction basin 2 at the
cover 25 is a function of the highest level L.sub.1P of the sea
directly above the water intakes 51 and 52, relative to the cover
25. The depressurization in the suction basin 2 will be more or
less significant, depending on the flow of water I.sub.P through
the calibrated openings 26. It is possible to dispense with the
openings 26 and replace them with valves that allow air to enter
and prevent water from exiting. In this case, the structures of the
basin 2, the cover 25, and the filtration system 12, must withstand
the added pressure.
[0096] The water intake installation further comprises a system for
supplying additional water that is functionally analogous to the
one described above with reference to FIG. 4, and that includes a
water duct in the form of a backup tunnel 30 connected to at least
one backup water intake 15 submerged in the sea. A backup water
intake 15 must be submerged at a depth that ensures it is never
exposed except in the case of an extremely exceptional drop in the
sea as can occur before the arrival of the first wave of a tsunami,
and therefore is located below the level L.sub.L of the lowest tide
during the strongest tidal coefficients. It is generally not
necessary for a backup water intake 15 to be arranged more than ten
meters below level L.sub.L, an arrangement of less than ten meters
below this level L.sub.L generally being sufficient to prevent
contamination of the water intake by floating objects or algae. A
main water intake 51 or 52 is generally arranged at more than
twenty meters below the level L.sub.L of the lowest tide, so that
the decrease in the maximum temperature of the water it draws is
significant. A backup water intake 15 will therefore usually be
positioned at a height H.sub.E of at least ten meters above a main
water intake.
[0097] The backup tunnel 30 passes under the dike 61 and comprises
a horizontal passage 35 which traverses a wall of the suction basin
2 to open into the basin at an end 35B that forms a vertical planar
surface. An obstructing device 9 in the form of an autonomous
pivoting valve, which may be virtually identical to the one
described above with reference to FIG. 4, is installed in the
suction basin 2, for example in a compartment 2B of the basin
providing maintenance access to the valve without the risk of
objects or workers being sucked into the main chamber 2A of the
suction basin. An opening 21 provided between the compartment 2B
and the chamber 2A may be equipped with a security grid. In the
closed position of the valve 9, the planar sealing panel 90 forming
the obstructing member of the valve is seated against the end 35B
of the backup tunnel 30 and thus closes off the water duct.
[0098] As represented in FIG. 18, in the case of an insufficient
supply to the pumping station 10 of water coming from the suction
tunnel, the water level L.sub.2 in the suction basin 2 drops until
the predetermined critical difference .DELTA.hV between the level
L.sub.1 of the sea and the level L.sub.2 of the basin is exceeded,
which causes the valve 9 to pivot and therefore opens the water
duct. The water coming from the sea through the backup tunnel 30
passes into the compartment 2B of the basin and then into the main
chamber 2A of the basin through the opening 21.
[0099] It is understood that the obstructing device of the system
for supplying additional water of FIG. 17 is not limited to a valve
9 with a massive counterweight means. For example, a valve device 9
as described above with reference to FIG. 11, FIG. 13, or FIG. 15,
may instead be provided in the compartment 2B of the suction basin,
with the passage 35 being suitably adapted.
[0100] As represented in FIG. 19, according to another embodiment
of the obstructing device, the pivoting valve device 16 comprises a
float device 96, arranged so as to be fully submerged in water
during a normal supply of water by the suction tunnel 3. The volume
of the float device 96 is defined beforehand so that the buoyancy
exerted on the fully submerged float is sufficient to keep the
valve 16 closed during a normal supply of water, by
counterbalancing the opening force of the valve due to the
differential water pressure exerted on the face of the sealing
panel 90 on the backup tunnel 30 side. The float 96 has a structure
adapted to withstand the high water pressure in the suction basin 2
in case of tidal waves.
[0101] In a case of insufficient water supply to the pumping
station 10, if the level of water L.sub.2 in the suction tank 2
falls sufficiently below the level L.sub.2L of lowest tide to reach
the predetermined trigger level L.sub.2V, the float 96 is designed
to emerge at least partially from the water, so that the decrease
in buoyancy exerted on the float causes the valve 16 and thus the
obstructing member 90 to pivot. Advantageously, the volume and
weight of the float device 96 are defined beforehand so that if the
critical difference in water level .DELTA.hV is exceeded, the valve
opening force due to the water pressure differential is greater
than the valve closing force due to the torque of the floating
device with respect to the pivot shaft 91. Thus, once the level
L.sub.1 of the sea is substantially above the level L.sub.L of the
lowest tide during the strongest tidal coefficients, the valve 16
begins to pivot to open the water duct as soon as the predetermined
critical difference in water level .DELTA.hV is exceeded.
[0102] A significant advantage of such a valve 16 with its float
device 96 lies in that it is virtually certain that the valve will
pivot autonomously, at the very latest shortly after the water
level L.sub.2 in the suction basin drops below the trigger level
L.sub.2V. Even assuming some seizing of the pivot shaft 91 or
adherence of the panel 90 to the end 35 of the passage due to
organic matter, the drop of the water level L.sub.2 to below the
trigger level L.sub.2V exposes the float 96 to the point where the
valve opening force inevitably becomes sufficiently strong to
overcome the static forces preventing pivoting. For example, with a
water level L.sub.2 as indicated in FIG. 19, one can see that the
valve 16 cannot remain closed and it pivots to open as represented.
It is understood that such a valve with float device may also be
used as an obstructing device in place of valve 9 in a system for
supplying additional water such as that of FIG. 4.
[0103] A possible disadvantage of the device lies in the limitation
to how far the valve can pivot, which may not allow sufficient flow
of water through the backup tunnel 30 if the production pumps of
the pumping station 10 are restarted during periods when the water
temperature at the sea's surface remains cold. In this case, one
solution would be to provide a sufficient cross-sectional area of
the backup tunnel 30 and the passage 35, and to have a controlled
valve appropriate for a large cross-sectional area in parallel with
the valve 16 which in turn may be arranged to simply allow a water
flow certain to be sufficient to supply the backup pumps of the
pumping station. In addition, the pivot shaft 91 may be formed by a
bar having a supporting ridge along its length as explained above
in relation with the embodiment shown in FIG. 11, which should
prevent significant seizing of the shaft without requiring special
maintenance.
[0104] Moreover, if the high water is due to a tsunami, and if no
significant earthquake before the tsunami is felt in the plant, it
may be desirable not to shut down the reactor units in the plant
and therefore not to shut down the production pumps in the pumping
station during the high water. A water intake installation such as
the one described above with reference to FIG. 17 and FIG. 18
allows such operation. However, as explained above, during this
period which may last several minutes, the supply of water to the
production pumps must then be able to occur solely from the reserve
of water contained in the suction basin 2 below the mouth 7E of the
passage 7. As a first approximation, it is estimated for example
that a reserve volume of water of up to about 100,000 m3 for a
pumping station of a reactor unit would be needed to overcome the
most critical drop conceivable in the level of the sea preceding
the first wave of a tsunami, lasting at least fifteen minutes. For
example, with a height of at least five meters between the bottom
2B of the basin 2 and the mouth 7E of the passage 7, it would take
about two hectares of basin surface area to ensure such a reserve
volume of water.
[0105] There are disadvantages to creating a suction basin such as
the one in FIG. 17, for the case of a particularly large reserve
volume below the level of the mouth 7E of the passage 7. First,
since the basin has a roof that forms a cover resistant to a water
pressure in the basin of for example about two bar in order to
contain the water in case of tsunami or tidal wave, the
implementation of such a roof to cover an area of a hectare or more
involves significant construction costs. This is even more true if
the suction basin 2 is shared by multiple pumping stations
supplying several reactor units, where the surface area of the
basin roof substantially increases the construction costs of the
water intake installation as a whole. Furthermore, since the
pumping rate of a pumping station when supplying a reactor unit in
full production is about 70 m3 per second for example, it would
take almost an hour at a flow rate of about 140 m3 per second to
refill completely a suction basin shared by two reactor units and
containing about 500,000 m3 measured as the high tide average.
Depending on the temperature of the outside air, especially if the
outside temperature exceeds 30.degree. C. in the shade, the water
flowing into the basin could grow warmer by about 1.degree. C. or
more between when it exits the suction tunnel and enters the
pumping station. A relative decrease in efficiency of the facility
may therefore occur during certain times of the year, in comparison
to a suction basin of much smaller volume.
[0106] To overcome these potential disadvantages, an embodiment of
a water intake installation of the invention proposes establishing
an emergency water reserve in a reserve basin containing a volume
of water which remains substantially unchanged while water is being
supplied normally to the suction basin by the suction tunnel or
tunnels.
[0107] An example of such an embodiment is represented in FIG. 20.
A reserve tank 20 is separated from the suction basin 2 by a dam
wall 80 in which is provided with an opening 85 forming a water
duct for the system for supplying additional water. The water duct
85 opens into the suction basin 2 in a curved side of the wall 80
forming a circular arc or some other continuous curve in a vertical
plane corresponding to the plane of the figure. An obstructing
device 17, shown in its closed position in the figure, comprises an
obstructing member in the form of a sealing panel 90' associated
with a supporting structure, the panel having an outer surface of a
shape substantially complementary to the curved side of the wall
80. The panel 90' with its supporting structure is connected to a
horizontal pivot shaft 91' on which it pivots to bring the
obstructing device 17 to a position which opens the water duct 85
as shown in FIG. 21. The pivot shaft 91' may substantially be
coincident with a straight line forming the central axis of
curvature of the curved side of the wall 80. Since the widest pivot
angle of the obstructing device 17 is less than 90.degree., and
here is even less than 45.degree., it may be arranged that the
pivot shaft 91 is formed by a bar having ridges along its length
that are in alignment with a same straight line and that face
towards opposite sides and press against concave mount surfaces,
thus providing a submerged pivot shaft that does not require
lubrication.
[0108] The outer surface of the sealing panel 90' is arranged to be
flush with the surface of the curved side of the wall 80 when the
obstructing device 17 is in the closed position, leaving only a
small gap allowing a limited flow of water to escape from the
reserve basin 20 to the suction basin 2 when the water duct 85 is
closed off. However, the gap between the sealing panel 90' and the
curved side of the wall 80 is sufficient to prevent any risk of the
panel catching on the wall, the thickness of the gap being able to
fluctuate for example with the thermal expansion of the supporting
structure of the panel. Too thin of a gap could allow contact where
the panel and the wall become jammed, preventing the obstructing
device 17 from opening.
[0109] The obstructing device 17 comprises a counterweight means
arranged on the side opposite to the obstructing member 90'
relative to the pivot shaft 91'. The counterweight means comprises
a main counterweight member 97 including a supporting structure
rigidly connected to the supporting structure of the panel 90'. The
obstructing device 17 is designed to begin pivoting from its closed
position as soon as the water level in the basin reaches a
predetermined trigger level L.sub.2V at which a substantial portion
of the main counterweight member 97 emerges from the water. The
main counterweight member 97 preferably weighs between 80% and 200%
of the weight of the obstructing member 90'. For example, a weight
approaching 200% of the weight of the obstructing member allows
placing the pivot shaft 91' and main counterweight member 97 closer
together, thereby reducing the overall size of the obstructing
device 17 and in addition allowing a wider pivot angle and thus a
wider opening of the device for a given decrease of the water level
in the suction basin. In addition, the counterweight means may
comprise an auxiliary counterweight movably mounted on the
supporting structure of the main counterweight. In addition, in
order to reduce the surface area of the suction basin floor,
thereby reducing the surface area of the roof forming the cover
device 25 of the basin, it is possible to install at least one
obstructing device 17 between two mouths 7E of two passages 7
connecting the tunnel 3 to the suction basin 2.
[0110] The floor of the reserve basin 20 extends over a much
greater surface area than the suction basin 2, and its top is open
to the outside. The reserve basin 20 does not require a waterproof
roof, although a system of protection against the sun's rays, for
example a tarpaulin, remains possible. The water level L.sub.3 in
the reserve basin 20 is kept relatively constant, below the cover
device 25 of the suction basin. For example, pumps to circulate
water in both directions between the suction basin and the reserve
basin may be provided, to compensate for the continuous leakage of
water into the suction basin through the obstructing device 17 or
conversely to discharge water into the suction basin during heavy
rains. The volume of water in the reserve basin 20 remains
substantially unchanged as long as the suction basin is being
supplied with water normally by the suction tunnel or tunnels. For
a nuclear power plant where the suction basin supplies water to two
reactor units, a reserve basin 20 containing for example about
100,000 m3 of water seems sufficient to overcome the most critical
drops conceivable in the level of the sea.
[0111] The difference in height between the water level L.sub.3 in
the reserve basin 20 and the water level L.sub.2 in the suction
basin 2 can be significant, particularly at low tide, and for
example can reach about ten meters at the lowest tide of the year
for an ocean. As a result, a differential water pressure on the
order of a bar at its peak is applied to the sealing panel forming
the obstructing member 90' between the reserve basin 20 and the
suction basin 2. In addition, the water duct 85 closed off by the
sealing panel 90' must have a sufficient cross-sectional area to
allow a flow of water enabling the production pumps of a pumping
station to continue to operate, for example about 70 m3 per second,
which implies a relatively large surface area for the sealing panel
90'. The forces generated by the differential water pressure on the
sealing panel 90' result in a force represented in FIG. 20 by a
vector F2 which is applied at or near the geometric center of the
surface of the sealing panel blocking the water duct 85. This force
vector F2 is directed perpendicularly to the central axis of
curvature of the curved side of the wall 80, which may be designed
to be coincident with the pivot shaft 91', such that the force
vector generates no torque on the sealing device 17.
Advantageously, the central axis of curvature of the curved side of
the wall 80 may be located somewhat above the pivot shaft 91', such
that the force vector F2 directed perpendicularly to this central
axis generates a torque on the obstructing device 17 that helps the
device to pivot open. This latter arrangement may be of interest
for reducing the weight necessary for the main counterweight member
97, as long as the volume of this member remains sufficient for the
buoyancy required when the obstructing device 17 is in the closed
position.
[0112] In the embodiment represented in FIG. 20, the system for
supplying additional water can provide indirect communication
between the suction basin 2 and a second emergency water reserve
consisting of the body of water 5, which is the sea in this
example. In the case where the supply of water to the suction basin
by the suction tunnel or tunnels becomes insufficient for a lasting
period, and in particular in the case of a critical collapse in the
suction tunnel or tunnels, a lasting solution must be implemented
for supplying water to the suction basin once the volume of water
in the reserve basin 20 has severely decreased. Given the proximity
of the sea, it is advantageous to provide a water duct in the form
of a backup tunnel 30 connected to at least one backup water intake
15 submerged in the sea, as described above in reference to FIG.
17. It is understood that if the plant is located near a water
source such as a river or lake providing the possibility of a
reliable and sustainable source for the second emergency water
reserve, a link for supplying water between such a water source and
the reserve basin 20 may possibly be preferred over the solution of
a backup tunnel 30. For example, a small artificial lake of
seawater maintained at a certain level by pumping water from the
sea could be provided at or near the site of the nuclear power
plant, at a height slightly above the reserve basin 20 and
connected to the reserve basin or directly to the suction basin
through a pipe closed off by a valve.
[0113] Given that the reserve basin 20 is not closed off by a cover
device, the obstructing device sealing the water duct created by
the backup tunnel 30 must not allow seawater to enter the reserve
basin in case of a tidal wave, because the reserve basin could then
overflow and risk flooding the plant. Therefore, a sealing device
such as the device 9 referenced in FIG. 17 is not appropriate for
the reserve basin 20. In addition, when the water level in the
suction basin 2 drops in a manner defined beforehand as abnormal,
it may be advantageous to detect the state of the sea's level to
determine whether the decreased level in the suction basin is
caused by the sea abnormally retreating. If the level of the sea
has not changed significantly, leading to the conclusion that a
critical collapse has occurred in the suction tunnel or tunnels,
the production pumps of the pumping station can be shut down and
switched over to the backup pumps. The volume of water in the
reserve basin 20 is usually enough to supply water to the backup
pumps for at least two hours. As this provides the time to open the
obstructing device blocking the backup tunnel 30, an obstructing
device in the form of a non-autonomous controlled valve, for
instance a gate valve, is possible. Unlike an autonomous valve,
such an obstructing device does not provide a passive safety
mechanism, and once the valve is open it must be possible to ensure
its closure in the event of a tidal wave.
[0114] An autonomous obstructing device similar to device 17 may be
used to close off the backup tunnel 30. Alternatively, a pivoting
float device 18 may be employed that does not require a
counterweight. The obstructing device 18 represented in FIG. 20
comprises a curved sealing panel 90' pivoting about a pivot shaft
91' which can be arranged to coincide with the straight line
forming the central axis of curvature of the curved face of the
panel. A float 98 is attached to the supporting structure of the
sealing panel and is adapted to push the structure upward as long
as the float is completely submerged. A small adjusting
counterweight can be added to the device, in order to adjust the
pivoting that is triggered when the float rises above the water
surface.
[0115] As represented in FIG. 21, during a critical drop in the
level of the sea preceding the first wave of a tsunami, the sea
withdraws to below the level L.sub.L of the lowest tide for a
period of several minutes. The level L.sub.2 of the water in the
suction basin 2 first drops very quickly because the water flows
back toward the passages 7 where the water level is attempting to
establish an equilibrium with the level L.sub.1 of the sea. The
rapid exposure of a large portion of the main counterweight member
97 of the obstructing device 17 greatly decreases the buoyancy
exerted on this member and causes an almost complete opening of the
closing device, allowing the reserve basin 20 to supply water to
the suction basin 2 in a limited flow but designed to be sufficient
for the production pumps if these have not been shut down. The
obstructing device 17 is arranged such that level L.sub.2
stabilizes at a height slightly below the mouths 7E of the passages
7, so that as little water as possible is lost from the reserve
basin through the passages 7. One will note that if level L.sub.2
climbs back up slightly, the obstructing device 17 pivots and
somewhat obstructs the water duct 85, which reduces the flow so
that level L.sub.2 can stabilize as represented in FIG. 21.
Furthermore, it may be advantageous to detect the state of the
sea's level in order to check whether the decreased level in the
suction basin is caused by an abnormal withdrawal of the sea. In
this case, and if no significant earthquake preceding the tsunami
was felt in the plant, it is not necessary to shut down the
production pumps which can continue to be supplied with water by
the reserve basin until the water returns to the suction basin via
the suction tunnel or tunnels. Even so, it may be decided when
designing the plant that the production pumps will be shut down
systematically in the event of an abnormally low water level in the
suction basin, thus limiting the volume required in the reserve
basin and therefore the construction cost of the basin.
[0116] When the first wave of the tsunami arrives, as represented
in FIG. 22, the sea can reach a level L.sub.1P located several
meters above the cover device 25 of the suction basin. The water in
the suction basin rises, which causes the obstructing device 17 to
close. Once the water in the suction basin has reached the cover
25, a limited flow of water I.sub.P is allowed to exit to the
outside environment through the calibrated openings 26. This flow
I.sub.P can be channeled to the reserve basin 20, where the water
level L.sub.3 is still far below the maximum capacity of the basin.
The obstructing device 18 which blocks the backup tunnel 30 is not
triggered to pivot by the differential water pressure applied to
its obstructing member 90', since the pressures result in a force
vector F2 directed toward the pivot shaft 91'. The operation of the
nuclear power plant can be continued in this tidal wave situation
during the period required for the sea to return to its normal
level, for example about half an hour.
[0117] In FIG. 23, one can see that a critical collapse has
occurred in the suction tunnel or tunnels in at least one collapse
area 55. The water level L.sub.2 in the suction basin 2 has dropped
which has caused the obstructing device 17 to open, significantly
draining the reserve basin 20 into the suction basin to achieve
substantially the same level L.sub.2. During this water transfer
period, the production pumps of the pumping station were shut down
and switched over to the backup pumps. The float of the obstructing
device 18 has been partially exposed above the surface of the
water, causing the partial opening of the obstructing device and
thus supplying the reserve basin 20 via the backup tunnel 30. The
partial opening of the obstructing device 18 adjusts automatically
to the water consumption of the pumping station, because if the
level L.sub.2 drops too much the obstructing device 18 opens
further until equilibrium is restored.
[0118] As represented in FIG. 24, the pivoting of an obstructing
device 17 or 18 to open it, and possibly also to close it, may
optionally be actuated by a trigger device 70 connected for example
to a control system associated with at least one water level
detection system. The trigger device 70 may, for example, comprise
a winch possibly on a crane, acting on a cable 71 connected to the
structure of the obstructing device. Such a trigger device has the
advantage of allowing the obstructing device to pivot automatically
if the winch is not activated. In the example shown in FIG. 24,
once the suction tunnel 3 is repaired and the suction basin 2 is
being supplied with water normally, the trigger device is actuated
to force open the obstructing device 18 in order to fill the
reserve basin via the backup tunnel 30 while the sea is at high
tide. Considering the situation of a critical collapse of the
suction tunnel 3 in reference to FIG. 23, one will note that the
installation of trigger devices 70 as represented in FIG. 24 would
allow keeping the obstructing devices 17 and 18 completely open if
it is desired to increase the flow of water between the backup
tunnel 30 and the suction basin 2, making it possible to restart
the production pumps.
[0119] In addition, during the design phase one could provide means
for securing the obstructing device 18 in its closed position, or
for removing the obstructing device 18 and sealing the water duct
formed by the backup tunnel 30. Once sufficient experience has been
obtained with the operation of nuclear power plants supplied with
water through reinforced suction tunnels, it is found out that a
critical collapse in a suction tunnel cannot reduce the flow of
water to the point that it impacts the water supply to the backup
pumps, it could be decided to temporarily or definitively block off
the water duct provided by the backup tunnel. In such a scenario,
it might even be possible to do without a backup tunnel in the
construction of new water intake installations of the invention
similar to the installation of FIG. 20. The proximity of the sea in
this case allows providing emergency solutions for supplying water
to the reserve basin 20 if so needed.
[0120] In FIG. 25, another embodiment of a water intake
installation according to the invention is represented that is
similar to the embodiment described above with reference to FIG.
23. These essentially differ in that the suction basin 2 is
directly supplied with water by a backup tunnel 31 connected to at
least one backup water intake 15 submerged in the sea. For the
purposes of the diagrammatic representation in FIG. 25, the backup
tunnel 31 is represented as passing through the reserve basin 20 to
end in a horizontal pipe 36 traversing a face of the dam wall 80
separating the reserve basin 20 from the suction basin 2. The
horizontal pipe 36 forms a water duct 86 distanced to a greater or
lesser extent from the water duct 85 associated with the
obstructing device 17. It may be preferred to have a backup tunnel
31 which does not traverse the reserve basin 20. Furthermore, as
explained above with reference to FIG. 24, the obstructing device
17 may be associated with a trigger device 70 comprising, for
example, a winch acting on a cable 71. The trigger device 70 is
connected here to a control system 50 associated with multiple
water level detection systems using water sensors 28 to detect
primarily whether the water level in the suction basin 2 has
dropped in a manner defined beforehand as abnormal, the measurement
of the rate of change of the water level possibly being a parameter
for determining an abnormal drop.
[0121] In order to close off the water duct 86 formed by the backup
tunnel 31, an autonomous closing device such as one or the other of
the obstructing devices 9 and 16 described above with reference to
FIG. 17 and FIG. 19 may be used, as this is the same configuration
of a closed suction basin that one must be able to place in
communication with the sea via a backup tunnel. With such an
obstructing device 9 or 16, the opening of the device will be
arranged to trigger for a level L.sub.2 higher than the
predetermined level L.sub.2V for triggering the opening of the
closing device 17 which blocks the water duct 85 between the
suction basin and the reserve basin, so that practically none of
the water in the reserve basin is used except when there is an
abnormal withdrawal of the sea.
[0122] An autonomous obstructing device such as device 9 or 16 is
not essential, however, and in particular it is conceivable to use
an obstructing device 19 which is only opened by a trigger device.
The lack of autonomy of such an obstructing device 19 does not
necessarily compromise the safety of the installation, and in
particular there can be redundancy in the trigger device assigned
to the obstructing device. In addition, the obstructing device 19
in the installation of FIG. 25 is preferably designed to open only
when a critical collapse in the suction tunnel or tunnels 3 has
occurred, and is intended to open before the water level L.sub.2 in
the suction basin reaches the predetermined level L.sub.2 V for
triggering the opening of the obstructing device 17. As a result,
if there is a malfunction in opening obstructing device 19, the
water level L.sub.2 in the suction basin continues to drop to the
predetermined level L.sub.2V, which triggers the opening of
obstructing device 17 automatically or by the associated trigger
device 70, thus supplying the suction basin from the reserve basin.
The volume of water in the reserve basin 20 is usually enough to
supply the pumps for backup operation for at least two hours, which
provides time to restore control to the opening of obstructing
device 19.
[0123] To ensure that obstructing device 19 is only opened in cases
of critical collapse in the suction tunnel or tunnels 3, we need to
be able to determine with certainty that a rapid drop in the water
level L.sub.2 of the suction basin is not due to a withdrawal of
the sea. To achieve this, the control system 50 may be associated
not only with a system for detecting a decrease in the suction tank
water level, but also a system for detecting a decrease in the
level of the sea. Each detection system, comprising for example
water sensors 28 at different heights for measuring the water
level, sends data 29 to an analysis system associated with the
control system 50. The analysis system is intended to determine if
the water level in the suction tank 2 is dropping in a manner
defined beforehand as abnormal and if the level of the sea has not
dropped abnormally. If both conditions are true, it is almost
certain that the suction tunnel or tunnels have suffered a critical
collapse. The control system 50 then sends a trigger command 59 to
a trigger device 70 to actuate opening obstructing device 19, for
example by pulling a cable 71 to unlock a locking system that keeps
obstructing device 19 closed. The trigger command 59 may also
initiate switching from the production pumps of the pumping station
to the backup pumps. As represented in FIG. 25, once the
obstructing device 19 is open, the backup tunnel 31 supplies water
to the suction basin 2 and the water level L.sub.2 rises to
stabilize at more or less the level L.sub.1 of the sea. One will
note that the water duct 86 formed by the backup tunnel 31 may be
lower than the representation shown in FIG. 25, and may for example
be located at the bottom of the reserve basin in the same manner as
water duct 85.
[0124] FIG. 26, as well as FIG. 27, FIG. 28, and FIG. 29, represent
different positions of a same obstructing device 19 and are
discussed together. The obstructing device 19 shown is an example
embodiment of a non-autonomous closing device which can be used in
the water supply system of the water intake installation of FIG.
25. In FIG. 26 and FIG. 28, the obstructing device 19 is
represented in its closed position. The device comprises an
obstructing member 90 in the form of a generally planar sealing
panel that is pivotable about a pivot shaft 91. The panel 90 closes
off the water duct 86 formed in a face of the dam wall 80. The
closed position is maintained by a locking system comprising
brackets 82 attached to the wall 80 and a locking bar 72 inserted
between the brackets 82 and a free end portion of the panel 90. The
locking bar 72 is connected to at least one cable 71 which can be
pulled by a trigger device 70 as described above. Rollers 73 may be
provided on either side of the locking bar 72 to facilitate
displacement of the bar when unlocking the device.
[0125] As represented in FIG. 27 and FIG. 29, actuation of a cable
71 pulls the locking bar 72 upward so that it no longer prevents
the sealing panel 90 from pivoting under the effect of the
differential water pressure applied on the face of the panel on the
reserve basin 20 side. The panel 90 is designed to pivot at least
90.degree. in order to completely unobstruct the water duct 86
formed by the backup tunnel. As represented in FIG. 30, it may be
arranged that the pivot shaft 91 of the sealing panel 90 is formed
by a bar 99 having a ridge along its length, for example a ridge
having an oval profile, the bar 99 pressing against a concave
surface of a mounting member 81 parallel to the bar 99 and fixed to
the wall 80. The contours of said ridge of the bar 99 and of the
concave surface of the mounting member 81 are shaped to allow the
panel to pivot at least 90.degree. without excessive jamming or
friction.
[0126] In FIG. 31, another embodiment of a water intake
installation similar to that of FIG. 20 exclusively uses
non-autonomous obstructing devices 19, meaning devices which are
only opened by a trigger device. The control system 50 is adapted
to control the opening of each obstructing device 19 individually,
and is associated with systems for detecting a water level decrease
in the suction basin 2 and in the reserve basin 20 using sensors 28
that detect the presence of water. An obstructing device 19 may
open irreversibly, meaning as was the case for the device 19
described above that it is not possible to close the obstructing
member 90 without performing a specific operation once open. It is
also possible for an obstructing device 19 to open reversibly, as
is the case for example for a butterfly valve or a gate valve.
[0127] If there is an abnormal drop in the water level L.sub.2 in
the suction basin, the first obstructing device 19, located between
the suction basin 2 and the reserve basin 20, is triggered open
while the second obstructing device 19 which blocks the backup
tunnel 30 remains closed. The trigger command 59 also causes a
switch from the production pumps of the pumping station to the
backup pumps. The cross-sectional area of the water duct opened by
the first obstructing device 19 is intended to be small enough that
the water level L.sub.3 in the reserve basin 20 does not drop too
quickly, but must allow sufficient flow, for example between 5 m3
and 15 m3 per second, so that while the production pumps are shut
down the water level L.sub.2 in the suction basin 2 remains only
slightly below the mouth E7 of a passage 7 connecting the suction
basin to the suction tunnel 3. The volume of water in the storage
basin 20 is intended to be enough so that, if the abnormal drop in
level L.sub.2 is due to a withdrawal of the sea, the supply of
water to the backup pumps is ensured until the sea returns to above
its lowest tide level L.sub.L, and the water level in the reserve
basin 20 remains above the level L.sub.4 that triggers the second
obstructing device 19. The reserve basin 20 is covered by a cover
device 25' provided with at least one calibrated opening 27, in
particular in the case of an obstructing device 19 that opens
non-reversibly, to prevent the reserve basin from overflowing and
flooding the plant in case of a tsunami.
[0128] If the abnormal drop in level L.sub.2 is due to a critical
collapse in the suction tunnel or tunnels 3, the water level in the
reserve basin 20 falls relatively slowly until it reaches the level
L.sub.4 that triggers the second obstructing device 19, and the
system for detecting a dropping water level in the reserve basin 20
issues a trigger command 59 to open the second obstructing device.
As a precaution, it is possible to order the second obstructing
device to open before level L.sub.4 is reached, once it is certain
that there has been a critical collapse in a tunnel 3. The reserve
basin is then supplied with water by the backup tunnel 30. The
water level L.sub.2 in the suction basin 2 and the water level
L.sub.3 in the reserve basin 20 climb back up to substantially the
level L.sub.1 of the sea. The operation of the pumping station of
the nuclear power plant in safe mode is thus ensured, even in the
case of another tsunami event.
[0129] As an alternative to the above embodiment, it is also
possible to connect the backup tunnel 30 to the suction basin 2
directly. The opening of the second obstructing device 19
associated with the backup tunnel 30 would then be ordered once it
is certain that the suction tunnel or tunnels 3 are more or less
blocked. In addition, a non-autonomous obstructing device such as
the obstructing device 19 described above with reference to FIGS.
26-30 may be used in place of an autonomous obstructing device in a
water supply system with no reserve basin, for example the water
supply system described above in reference to FIG. 17, instead of
the valve device 9. In this case, if the obstructing device 19 must
be opened at some point, the device must be returned to the closed
position before the production pumps are restarted once the suction
tunnel or tunnels 3 are operational, to avoid the water coming from
a suction tunnel being heated by the water coming from a backup
tunnel 30.
[0130] A water intake installation according to the invention can
be intended for equipping a nuclear power plant separated from the
sea by land unsuitable for construction or by a wide strip of dunes
or other irregularities that descend in the inland direction, to
mean sea level or below. It is understood that a suction basin of
the installation must be shaped so that the basin floor is below
mean sea level and at least several meters below the lowest tides
for bodies of water having tides. Depending on the suitability for
construction and/or the topology of the land along the coast, it is
possible to construct the nuclear power plant at a site some
distance from the shore, for example up to about five kilometers
away, taking into consideration the increased construction costs of
a suction tunnel for an installation with a tunnel of such
length.
[0131] If the coastline may experience exceptional tidal waves such
as tsunamis, a nuclear power plant having a water intake
installation such as one of the installations described above with
reference to FIGS. 17 to 31 can be installed away from the
shoreline, lengthening each suction tunnel and each backup tunnel
accordingly. In other cases, where there is no such risk of tidal
waves, a water intake installation as described above with
reference to FIG. 17 but without the cover device for the suction
basin may be used.
[0132] For reasons concerning the construction costs and
maintenance of the water intake installation, or for safety reasons
in areas of seismic activity, it may be advantageous to dispense
with a backup tunnel for such a plant established at a distance
from the shoreline, as long as there is an auxiliary source of
water such as a river or lake for example. In such cases, an
emergency water reserve may be provided, comprising a reserve basin
which can supply water to the suction basin of the installation by
a system for supplying additional water as described above.
[0133] As represented in FIG. 32, such an embodiment of a water
intake installation according to the invention, intended for a
nuclear power plant separated from the shoreline by a strip of land
Z unsuitable for construction, comprises a reserve basin 20 which
can be placed in communication with the suction basin 2 via a water
duct 86 formed in a wall 80 separating the two basins. The water
duct 86 here is closed off by a non-autonomous obstructing device
19, controlled by a control system 50 associated with a system for
detecting a decrease in the suction basin water level.
Alternatively, an autonomous obstructing device may be used such as
one of the passively activated obstructing devices 9, 16, 17, and
18 described above. Regardless of the type of obstructing device,
the device must open when there is an abnormal drop in the water
level L.sub.2 in the suction basin, and at the latest when the
water level L.sub.2 has dropped below the lowest tide level
L.sub.2L down to the predetermined trigger level L.sub.2V.
[0134] In the embodiment represented, water sensors 28 measure the
rate of change of the water level. If the level is dropping at a
rate exceeding a predetermined threshold greater than the highest
known normal rate of change in the tide level, this event is
characteristic of an abnormal condition indicating either an
obstruction or blocking of the suction tunnel or tunnels 3 or an
abnormal withdrawal of the sea. Once the abnormal condition is
detected, the control system 50 sends a trigger command 59 to a
trigger device, not shown in the figure, to actuate the opening of
the obstructing device 19. The control system 50 also controls the
shutdown of electricity production by the reactor unit or units
associated with the suction basin 2, and the switch from the normal
production pumps of the pumping station 10 to the backup pumps.
[0135] In a situation of normal production of electricity by the
plant as represented in FIG. 32, the obstructing device 19 closes
off the water duct 86 and thus prevents the water in the suction
basin from being heated by the water in the reserve basin when the
latter is warmer, especially in summer. The water level L.sub.3 in
the storage basin 20 is kept relatively constant and close to
completely filling the basin, for example at a height exceeding the
highest tide level L.sub.H, so that in case of heavy rainfall the
surplus water in the reserve basin overflows to the suction basin 2
where the level L.sub.2 is lower. The volume of water in the
reserve basin is intended to be sufficient to supply the backup
pumps for a predetermined emergency period after the production
pumps have been shut down, for example at least four hours.
[0136] During the predefined emergency period, and according to a
set procedure, arrangements are quickly made to supply water to the
reserve basin, or to the suction basin directly, by an auxiliary
water source such as a river 5'. The average flow of water that can
be drawn from the auxiliary source must be greater than or equal to
the pumping rate of the backup pumps. For example, taking enough
water to ensure an average flow rate of at least 5 m3 per second of
water is usually sufficient in most nuclear power plants to meet
the needs of a pumping station of a reactor unit which has stopped
producing electricity.
[0137] The water can be drawn from the river 5' for example using
an auxiliary pumping station 10' located at the edge of the reserve
basin 20 and connected to the river 5' by underground piping. The
pumps of the auxiliary pumping station 10 are advantageously
started up shortly after the obstructing device 19 is opened, in
order to maintain in the reserve basin 20 a level L.sub.3 that is
close to the fill level of the basin. In this manner, even if a
long term problem arises with drawing water from the river 5', for
example a failure in the auxiliary pumping station 10', the plant
personnel has a period of several hours to take appropriate
measures to restore an adequate water supply for the backup
pumps.
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